1. Field of the Invention
The present invention relates to a transmission for a vehicle and more particularly to an automobile transmission which can be assembled using components shared among a plurality of transmission types.
2. Discussion of Prior Arts
A transmission for transmitting engine power to driving wheel and for changing engine rotational speeds, is connected with a crankshaft of an engine through a clutch and has a main shaft and a counter shaft provided in parallel with the main shaft. The main shaft is provided with a plurality of driving gears and the counter shaft is provided with a plurality of driven gears meshing with the driving gears. When a driver operates a shift lever, a power transmission path obtained by a gear set of a driving gear and driven gear is established.
There is no difference in the basic construction between a transmission for front wheel drive vehicle and one for four wheel drive vehicle.
In case where the engine is mounted in the longitudinal direction of the vehicle, since the transmission is connected with the rear end of the engine, a power unit of the combination of the engine and transmission is required to reduce its own length so as to secure a space utility of the passenger compartment of the vehicle.
To reduce the length of such a power unit, Japanese Patent Application Laid-open No. Toku-Kai-Hei 1-156134 discloses a drive train for automobile in which a longitudinally mounted engine is slanted in the widthwise direction and a final reduction gear unit or a differential separated from the transmission is disposed on the slanted side of the engine. In this case, the axle shaft penetrates the side wall of an oil pan and crankcase of the engine. Further, Japanese Patent Application Laid-open No. Toku-Kai-Hei 7-167257 discloses a technique in which the final reduction gear unit is integrated with the transmission. This type of drive train is characterized in a simple layout and a good mountability.
On the other hand, a multiple speed ratio transmission is required in market from the view point of vehicle performance. Particularly, in case of four wheel drive vehicles, a multiple speed ratio transmission or a dual range transmission is strongly requested. For example, Japanese Patent Application Laid-open No. Toku-Kai-Shou 55-4293 proposes a dual range type transmission having a sub-transmission between the crankshaft and the main shaft so as to change over the rotation speed of the crankshaft into two stages, Low and High, with respect to the main shaft.
In order to raise a productivity of transmissions, it is required that components of the transmission are shared between a plurality of models or body types. For example, it is required that components common to both front wheel drive vehicles and four wheel drive vehicles are used on assembling transmissions.
In response to these requirements, as disclosed in Toku-Kai-Hei 1-156134, in case where the final reduction gear unit is disposed on the side of the engine, the axle shaft penetrates the side walls of the oil pan and the crankcase. Accordingly, an attempt to apply a dual transmission mechanism to this type transmission elongates the longitudinal size the transmission and as a result the layout of the drive train becomes more complicated.
Further, as shown in Toku-Kai-Hei 7-167257, in case where the final reduction gear unit is integrated with the transmission, the layout of the drive train is simplified, however since generally a front drive shaft is fitted to the hollow counter shaft, the length of the transmission increases. It is disadvantageous in length that a multiple speed ratio gearing mechanism or a dual range mechanism is further incorporated into the transmission.
It is an object of the present invention to provide a transmission whose drive train layout is simple, even in case where a multiple speed ratio transmission mechanism or a dual range mechanism is incorporated into the transmission. It is another object of the present invention to provide a compact transmission easy to be mounted on a vehicle. It is further object of the present invention to share components among a plurality of transmission types in order to reduce manufacturing cost of the transmission.
To achieve these objects, the present invention comprises a main shaft connected with a crankshaft of an engine through a clutch and having a plurality of first drive gears, a counter shaft provided below and in parallel with the main shaft and having first driven gears meshing with the first drive gears and having a second drive gear, a drive shaft provided below and in parallel with the counter shaft and having a second driven gear meshing with the second drive gear, a front differential provided at a front end of the drive shaft, a transmission case for accommodating the main shaft, the counter shaft, the drive shaft and the front differential, a space formed in a front part of the transmission case, a center differential provided at a rear end of the counter shaft for distributing driving force into driving force for front wheels and driving force for rear wheels, a sub transmission accommodated in the space for transmitting driving force of the engine to the main shaft while reducing the rotation speed of the engine.